Conventionally, a chain saw for cutting trees or timbers is provided with a loop-type chain cutter stretched between a driving roller to be driven by a motor and an idle pulley. Particularly, a conventional type of chain saw body is large-sized because of incorporating thereinto a motor as an integral device. In addition, when overload is applied to the chain cutter, there is a growing danger that the chain cutter or the chain saw body may be damaged. Further, the disadvantage is that a conventional type of chain saw is not available for cutting smaller trees or the like because of such a large-sized device. In order to remove the aforegoing disadvantages, the present invention has been achieved.